1. Field of the Invention
The invention relates to an exhaust gas purification catalyst useful as a three-way catalyst, and in particular, to an exhaust gas purification catalyst that significantly reduces discharge of hydrogen sulfide.
2. Description of the Related Art
Three-way catalysts are widely used as catalysts that reduce HC, CO and NOx in exhaust gas from automobiles. Such a three-way catalyst is obtained by having platinum group noble metals, such as Pt and Rh, carried by porous oxide carriers, such as alumina, ceria, zirconia, or ceria-zirconia. The three-way catalyst oxidizes HC and CO for purification, and also reduces NOx. These reactions proceed most efficiently in the atmosphere in which the amount of oxidation components is substantially equivalent to the amount of reduction components. Thus, in an automobile equipped with a three-way catalyst, air-fuel ratio control is performed so that fuel is combusted near the stoichiometric air-fuel ratio (A/F≈14.6±0.2).
With regard to the three-way catalyst, however, there has been a problem that sulfur oxide in exhaust gas is reduced and discharged as H2S when the atmosphere of exhaust gas swings to the reduction side. For example, alumina is an essential component of the three-way catalyst, while there has been a problem that in an automobile equipped with a three-way catalyst using alumina, H2S is formed when the catalyst temperature is high, that is, equal to or higher than 350° C. in a rich atmosphere. The mechanism of formation of H2S can be explained as follows.
SO2 in exhaust gas is oxidized into SO3 or SO4 with the help of the catalyst in a lean atmosphere. SO3 and SO4 are adsorbed by the basic sites of alumina and the SO3 and SO4 adsorbed are gradually concentrated on alumina. Then, in a rich atmosphere, SO3 and SO4 are reduced and H2S is formed. Because even a little amount of H2S is smelled by a person and causes discomfort, discharge of H2S should be suppressed.
In recent years, ceria, ceria-zirconia mixed oxide, or the like is used as a component of the carrier for the purpose of suppressing fluctuations in the air-fuel ratio. Because ceria has the capability of adsorbing and releasing oxygen, that is, ceria adsorbs oxygen in a lean atmosphere and releases oxygen in a rich atmosphere, it is possible to stably keep the exhaust gas atmosphere near the stoichiometric air-fuel ratio. However, because the basicity of ceria is higher than that of alumina, sulfur oxide tends to be adsorbed by ceria rather than alumina and ceria has the opposite effect in terms of the suppression of discharge of H2S. Specifically, the improvement of the capability of adsorbing and releasing oxygen with the use of ceria and the suppression of formation of H2S are in a mutually exclusive relation.
It is conceivable that oxide of Ni or Cu is additionally used as a component of the three-way catalyst. Oxide of Ni or Cu turns SO2 into SO3 or SO4 in an oxidizing atmosphere and stores sulfur as sulfide, such as Ni3S2, for example, in a reducing atmosphere, so that it is possible to suppress formation of H2S.
In Japanese Patent Publication No. H08-015554, for example, an exhaust gas purification catalyst is described in which noble metal is carried by a carrier including a mixed oxide of nickel oxide and barium oxide (hereinafter also referred to as nickel-barium mixed oxide), alumina, and ceria. With regard to this carrier, alumina and ceria capture sulfur oxide as sulfate in a lean atmosphere, and nickel-barium mixed oxide captures H2S in a rich atmosphere. Thus, it is possible to suppress discharge of H2S.
Published Japanese Translation of PCT application No. 2000-515419 (JP-A-2000-515419) and Japanese Patent No. 2598817 describe that formation of H2S is suppressed with the use of a carrier in which NiO, Fe2O3, etc. are mixed. Japanese Patent Application Publication No. H07-194978 (JP-A-07-194978) describes that formation of H2S is suppressed with the use of a carrier in which Ni and Ca are carried.
However, because Ni and Cu are environmental load substances, using these substances in exhaust gas purification catalyst for automobiles is gradually being restricted. When barium or the like is added to the three-way catalyst, there is a possibility that the original purification performance is deteriorated.
Japanese Patent Application Publication No. S63-236541 (JP-A-S63-236541) describes an exhaust gas purification catalyst in which noble metal is carried by a canter that contains oxide of at least one metal that is selected from Ti, Nb, V, Ta and Mn in addition to alumina and ceria, and also describes that it is possible to suppress formation of H2S.
In addition, there are the following published official gazettes as the most relevant related art. Japanese Patent Application Publication No. 2007-090254 (JP-A-2007-090254) discloses an example of double layer coated catalyst in which an upper catalyst layer contains zirconia particles that carry Rh and has no Pt, a lower catalyst layer contains ceria particles that carry Pt, and the entire amount of ceria contained is 49 g/L.
Japanese Patent Application Publication No. 2004-298813 (JP-A-2004-298813) discloses a double layer coated, exhaust gas purification catalyst that has an upper catalyst layer in which Rh is carried by less-thermally-deteriorative ceria-zirconia mixed oxide or porous alumina and that contains 50 to 70 wt % of less-thermally-deteriorative ceria-zirconia mixed oxide in which the weight ratio between ceria and zirconia is approximately 3:7. The same publication describes that a lower catalyst layer has alumina that carries Pt and also has oxygen-storing ceria-zirconia mixed oxide and that the lower catalyst layer contains 50 to 70 wt % of oxygen-storing ceria-zirconia mixed oxide in which the weight ratio between ceria and zirconia is approximately 1:1.
Japanese Patent Application Publication No. 2007-111650 (W-A-2007-111650) discloses an exhaust gas purifying catalytic converter capable of suppressing generation of odor without reduction in the efficiency of purifying exhaust gas. This includes two exhaust gas purifying catalytic converters that are arranged on the upstream side and the downstream side, respectively, and is characterized in that the ratio of the amount of the component that adsorbs oxygen on the downstream side to the amount thereof on the upstream side is within the range between approximately 1 and 0.5. The listed example contents of ceria in the three-way catalyst on the downstream side in an example are 30 g/L and 20 g/L.
The catalyst described in Japanese Patent Application Publication No. 2007-090254 (JP-A-2007-090254) contains a large amount of ceria and contains no stabilizing agent. Thus, although a high capability of storing oxygen is obtained when the catalyst is fresh, it is expected that the amount of H2S formed is large. In addition, it is expected that grain growth of ceria is significant and the rate of reduction in the capability of storing oxygen after durability tests is significant because neither zirconia nor an additive is contained.
The catalysts disclosed in JP-A-2004-298813 and JP-A-2007-111650 contain no stabilizing agent and no explicit description on the kind and amount is made in these publications. Thus, it is expected that grain growth of mixed oxide including ceria is significant and the rate of reduction in the capability of storing oxygen is significant.